Electrolytic process for preparing stannous fluoride



United States Patent 3,390,064 ELECTROLYTIC PROCESS FOR PREPARING STANNOUS FLUORIDE Gotlibs Baltakmens, Wilmington, Del., and John P. Tourish, Swarthmore, Pa., assignors to Allied Chemical Corporation, New York, N.Y., a corporation of New York Filed Oct. 6, 1964, Ser. No. 401,926 3 Claims. (Cl. 204-94) This invention relates to a process for the electrolytic production of stannous fluoride, and more particularly, to the production of stannous fluoride free from substantial contamination with stannic fluoride.

Stannous fluoride is an article of commerce with well established uses, one of which is its incorporation into fluoride-containing tooth pastes. For such purposes the stannous fluoride must meet rigid purity specifications and must be free from substantial contamination with stannic fluoride. Stannic tin contents of less than 3% are often specified.

Stannous fluoride has been produced in the past from stannic oxide by dissolving the oxide in hydrofluoric acid and recovering the resulting stannous fluoride. This is an expensive process, since stannous oxide is produced from metallic tin. If a satisfactory process could be devised for using tin metal to produce stannous fluoride, considerable savings could be effected.

Metallic tin, however, is relatively insoluble in =hydrofluoric acid and cannot readily be converted to stannous fluoride by reaction with HF except by the use of expensive procedures, for example, the action of anhydrous HF on finely divided tin powder, and the exercise of careful temperature control to prevent fusion of the powdered tin, e.g., maintaining refluxing HP at C. or below. Thus prior attempts to utilize metallic tin have left something to be desired.

Our early efforts aimed at producing stannous fluoride by the electrolysis of hydrogen fluoride solutions using a tin anode were unsuccessful in producing a stannous fluoride of sufficiently low stannic fluoride contents to meet the rigid specifications in this regard.

This difiiculty appears to be due to conditions obtaining in the vicinity of the anode which promote or accelerate the oxidation of the lower valence Sn, which originally forms, to the higher valence state Sn. We have found that such conditions appear to include the presence of anode slime which forms over the top of the tin anode in an electrolyte of aqueous hydrofluoric acid.

It is an object of the present invention to produce stannous fluoride having a low stannic fluoride content.

Another object of the invention is to provide a process for the electrolytic production of stannous fluoride wherein an exceptionally pure product is produced.

A further object of the invention is to provide a process for preparing stannous fluoride from metallic tin and hydrofluoric acid solution.

A still further object of the invention is to provide a process for the continuous production of stannous fluoride by the electrolysis of aqueous hydrofluoric acid solutions between a mercury cathode and a tin anode.

These and other objects are accomplished according to our invention wherein a solution of hydrofluoric acid is electrolyzed between a mercury cathode and a tin anode, the latter having superimposed thereon a thin coating of mercury-tin amalgam. The presence of the amalgam appears to prevent formation of anode slime and to prevent the oxidation of the stannous fluoride initially formed, to the stannic state. During electrolysis,

"ice

in the presence of the amalgam coated anode, the tin dissolves from the amalgam, free of any insoluble slime. The mercury of the amalgam does not dissolve, but simply serves as a transfer point for the tin about to go into solution. A small amount of mercury will etfectively coat a tin pig or sheet, the percentage of mercury increasing progressively until the anode is completely consumed.

The mercury is applied to the tin anode either by dipping the tin directly into metallic mercury or by applying a solution of a mercury salt to the tin such as mercuric nitrate, whereupon reduction to mercury occurs. A single dipping with a few minutes soaking will reduce enough mercury to give effective coating. After dipping, the anode is washed off with water to remove any excess salt.

When using elemental mercury to form the amalgam, a coating of one gram of mercury per square foot (0.09 sq. m.) is adequate although not critical. In general, an amalgam coating as thin as about 0.2 micron is satisfactory.

In carrying out the process of our invention, we use any suitable electrolytic cell, preferably a cell such as is disclosed and claimed in our copending application Ser. No. 265,211, filed Mar. 14, 1963, and now Patent No. 3,300,397, and shown in the annexed drawing.

The attached drawing is a schematic representation of apparatus suitable for carrying out continuous electrolytic production of stannous fluoride in accordance with a preferred modification of the present invention.

Referring to the drawing, there is shown a main electrolytic cell 1. The cell may be fabricated from iron or other suitable material. When the cell is made of a material which conducts electricity and/ or is subject to corrosion by the contents of the cell, it may be provided with a plastic lining composed, for example, of polyethylene, polypropylene, polystyrene, unplasticized polyvinyl chloride, etc. Cell 1, having a suitable support 2, is provided with a cathode 3, comprising a container 4, having a pool of mercury 5, and cathode lead 6, immersed in the mercury. The cathode lead must have good conduction and may be any metal which does not amalgamate to substantial extent, e.g., iron. The bottom 4a of the cathode container is a porous, woven material which is inert to hydrofluoric acid solution but offers low resistance to the electrolytic reaction. The material is composed preferably of linear polyethylene. The sides of the cathode container are relatively rigid and are made preferably of non-porous plastic material which is inert to the acid solution. Typical materials include polyethylene, polystyrene, polypropylene, polytetrafluoroethylene, metals coated with unplasticized polyvinyl chloride, etc. An anode 7, composed of tin in stick, sheet or other suitable form, covered with a thin film of mercury-tin amalgam 7a, is disposed near the bottom of cell 1 and is provided with anode lead 8. The anode lead can be a threaded carbon rod screwed into a carbon slab 8a at the bottom of the cell. Alternatively, the anode lead can be com osed of any suitable metal provided with acid-resistant insulation, e.g., polyethylene.

In operation, hydrofluoric acid solution is introduced through line 9 to form the electrolyte in cell 1. The electrolyte entering the cell may be heated, and/or external heat may be applied to the cell to attain the desired temperature, i.e., about 40 C. to 60 C. Upon passing an electric current through the electrolyte, tin ions from the anode react with the hydrofluoric acid solution to produce stannous fluoride solution at the anode: surface. Hydrogen is evolved at the cathode. As the stannous fluoride solution increases in strength, the gravity difference between it and the hydrofluoric acid solution results in formation of a layer 11 of the stannous fluoride solution at the lower part of the cell. The hydrofluoric acid solution forms a layer 12 above the stannous fluoride solution. The concentrated stannous fluoride solution is withdrawn through line 13 by means of pump 14, while hydrofluoric acid solution is added to the cathode container via line 9. The stannous fluoride solution is collected in receiver 15.

The aqueous stannous fluoride solution withdrawn from the cell can be isolated and recovered by any desired means, for example, by concentration of the solution followed by cooling to effect crystallization. The resulting recovered stannous fluoride product has a stannic tin content less than about 1%, usually of about 0.3%0.6% based on 100% stannous fluoride, whereas under the same conditions, using a tin anode without the amalgam coating stannic contents in excess of about occur, often as high as 9% or higher.

In continuous operation, the mercury of the cathode can be purified by continuously withdrawing it from cathode container 4 via line 16, and permitting it to flow into a small auxiliary electrolytic cell 17, fabricated from the same material used for cell 1, where the pool of mercury becomes an anode 18. The mercury has anode lead 19 immersed in it. Cell 17 is also provided with cathode plate 21, which is preferably carbon but may also be composed of any suitable metal such as tin. Plate 21 is provided with cathode lead 22. The cell contains a suitable electrolyte 20, e.g. hydrofluoric acid solution. Agitation in the cell is provided by means of agitator 23, to prevent mercurous oxide formation. Upon application of electric current to cell 17, tin from the mercury is plated out .on cathode plate 21. The amount of tin removed from the cathode mercury can be varied by changing the current on the power source, and the temperature of the cathode mercury can be controlled by changing the pumping rate. Purified mercury is con- .tinuously sent via line 24 by means of pump 25 into washing tower 26, in which cold water is continuously introduced through line 27 and withdrawn through line 28. The cooled purified mercury is returned via line 29 to cathode container 4 of the main electrolytic cell.

Hydrofluoric acid solution concentration should be at least about 5%, preferably not more than about 20%, suitably between about 9% and 15%. At lower HF concentrations, greater quantities of SnF are soluble in the solution than at higher concentrations. The concentration :of SnF which is withdrawn from the cell, can be any desired strength up to the saturation point in the particular HF solution employed. Higher concentrations are more productive and simplify the subsequent recovery of SnF in the solid state and hence are preferred. However, withdrawal of SnF solution before it reaches the saturation point is desirable in order to prevent premature crystalliz-ation. In general, operating at between about 9% and about 15% concentrations, at cell temperatures between about 50 C. and about 60 C., concentrations of SnF removed will preferably range between about 20% and about 40%.

Current densities are not unduly critical and may suitably be in the range between about 100 and about 300 amps/sq. foot at the cathode and between about 20 and about 60 amps/sq. foot at the anode.

The following specific example further illustrates our invention. Parts are by weight except as otherwise noted.

EXAMPLE 1 A reactor of the character shown in the drawing, having a polyethylene lining, was charged with a tin anode which had been treated with mercury by immersing it in liquid mercury, to give a coating of mercury-tin amalgam on its surface. The cell was filled with a aqueous solution of hydrofluoric acid, to a depth of 6 inches (about 3 gallons). A mercury cathode in a polyethylene cup with a polyethylene cloth bottom was fixed about 2.5 inches above the top of the tin anode. Current was turned on and maintained at.30 amps at an average of 5 volts after the solution had warmed up to about 50 C. The current density on the cathode was 158 amps/sq. ft. and on the anode was 29 amps/sq. ft. Stannous fluoride was thus produced and dissolved in the hydrogen fluoride.

Due to the higher specific gravity of the SnF solution, it remained at the "bottom of the cell above the anode. When the anode layer had built up to about 23% strength, it was continuously withdrawn at a rate of 300 cc. per :hour, the stannous fluoride concentration averaging about 23% during the course of the run. At the cathode area, 10% hydrofluoric acid was added at the rate of 225 cc./hour. The cell was run in this manner for one week, producing a solution with a stannic fluoride content of between about 0.3% and about 0.4% The surface of the anode remained clean and bright throughout the run, showing no evidence of anode slime.

Recovery of SnF Percent stannic Tin A stannous fluoride product produced by electrolysis in the same cell and under the same conditions as described above, cxcept that a plain (non-amalgam coated) tin anode was employed has a stannic tin content of about 9%.

While the above describes the preferred embodiments of our invention, it will be understood that departures can be made therefrom within the scope of the specification and claims.

We claim:

1. A method for producing stannous fluoride free from substantial contamination with stannic fluoride, which comprises passing an electric current between 'a mercury cathode and an anode of metallic tin having superimposed thereon a thin layer of mercury-tin amalgam, said cathode and anode being immersed in an aqueous hydrofluoric acid solution.

2. A method for producing stannous fluoride free from substantial contamination with stannic fluoride, which comprises passing an electric current between a mercury cathode and an anode of metallic tin havingsuperimposed thereon a thin layer of mercury-tin amalgam, said cathode and anode being immersed in an aqueous hydrofluoric acid solution of concentrations between about .9.% and about 15%. I

3. A continuous method for producingstannous'fluoride free from substantial contamination with stannic fluoride which comprises introducing an aqueous hydrofluoric acid solution into an electrolytic cell provided with a mercury cathode near the top thereof and a tin anode near the bottom thereof, the latter having superimposed thereon a thin layer of mercury-tin amalgam, passing an electric current between said anode and said cathode, continuously feeding hydrofluoric acid solution to the top portion of the cell and continuously withdrawing stannous fluoride solution from the bottom portion thereof.

(References on following page) 5 6 References Cited FOREIGN PATENTS UNITED STATES PATENTS 212 2/1892 Great Britain.

1,597,653 8/1926 Liltlf: 204-94 2 7 7 1 5 Lowe at al 204 94 JOHN MACK: y Examlner- 3,300,397 1/1967 Baltakmns et a1. 204-94 5 H. M. FLOURNOY, Assistant Examiner. 

1. A METHOD FOR PRODUCING STANNOUS FLUORIDE FREE FROM SUBSTANTIAL CONTAMINATION WITH STANNIC FLUORIDE, WHICH COMPRISES PASSING AN ELECTRIC CURRENT BETWEEN A MERCURY CATHODE AND AN ANODE OF METALLIC TIN HAVING SUPERIMPOSED THEREON A THIN LAYER OF MERCURY-TIN AMALGAM, SAID 